The present invention relates generally to photovoltaic modules and, more particularly, to photovoltaic modules having specularly reflective surfaces disposed alongside a plurality of photovoltaic cells to concentrate radiation onto the cells.
Photovoltaic cells are typically connected together in modules to provide electrical power at useful voltage and power levels. The output power produced by a module is directly related to the total radiation impinging upon its cells, including both radiation incident directly on the cells and any radiation reflected and/or refracted onto them by external means.
Designers of photovoltaic power systems have sought to enhance the power produced by a given number of cells by increasing the amount of radiation reaching the cells. Many attempts to do so have made use of three-dimensional mirrors or prism structures placed in front of the light-receiving surface of a photovoltaic cell to "concentrate" incoming radiation. Examples of such systems are found in the following U.S. Pat. No. 3,427,200 to Lapin et al.; U.S. Pat. No. 4,002,499 to Winston; U.S. Pat. No. 4,053,327 to Meulenberg, Jr.; U.S. Pat. No. 4,316,448 to Dodge; U.S. Pat. No. 4,440,153 to Melchior; and U.S. Pat. No. 4,711,972 to O'Neill. The systems of the Lapin et al., Winston, Dodge, and Melchior patents use mirrors extending outwardly from a plane containing the light-receiving face of one or more cells to reflect additional radiation onto the cells. O'Neill and Meulenberg, Jr. disclose the use of refractive covers disposed in front of one or more photovoltaic cells to bend incoming light. They can take the form of a cylindrical lens which captures radiation over a large area and focuses it onto a group of cells (O'Neill, FIG. 5), or a smaller cover placed directly against the face of an individual cell to bend radiation away from grid lines on the cell (O'Neill, FIGS. 1-4 and 6 and Meulenberg, Jr.). Unfortunately, many of the foregoing systems are bulky and expensive to fabricate. Their mirrors or prisms also tend to become obscured by dust and dirt, drastically reducing module efficiency.
Another method of directing additional radiation onto photovoltaic cells is to provide light diffusive surfaces between, above or below the cells, as disclosed in the following U.S. Pat. No. 3,971,672 to Lampkin; U.S. Pat. No. 4,116,718 to Yerkes et al.; U.S. Pat. No. 4,162,928 to Shepard, Jr.; U.S. Pat. No. 4,246,042 to Knasel et al.; U.S. Pat. No. 4,321,417 to Kurth et al.; and U.S. Pat. No. 4,493,942 to Sheng et al. Of these, the Yerkes et al., Shepard, Jr., Knasel et al. and Kurth et al. patents disclose the use of substantially planar diffusive reflectors between the cells of a flat panel photovoltaic module to direct incoming radiation back into a front cover for internal reflection onto an adjacent cell. However, the increase in current achieved in this way is rather small because only a minor portion of the radiation incident on the diffusive reflectors is reflected at an angle suitable for subsequent internal reflection.
Therefore, it is desirable in many applications to provide an improved, low-cost structure for significantly increasing the output of photovoltaic cells by concentrating solar radiation thereon.